BoilerFaultCodesThe 6 most common faults
01
H 9 High Engineer
Your boiler has detected an internal communication error between its electronic control boards or an incorrect configuration.
Full guide for H 902
L 9 High Engineer
Your boiler has overheated, causing it to shut down automatically for safety.
Full guide for L 903
HE High Engineer
Your boiler's internal computer has encountered a critical communication error and can no longer control the heating system safely.
Full guide for HE04
H9 High Engineer
Your boiler has shut down because the exhaust gases have reached a temperature that is too high, which is a safety feature to prevent damage to the system.
Full guide for H905
L9 High Engineer
Your boiler has sensed that the exhaust gases are getting too hot and has shut down as a safety precaution to prevent internal damage.
Full guide for L906
H n Medium Engineer
Your boiler is displaying a service notification or a specific lockout state that requires a professional to check the internal sequence and settings.
Full guide for H non the Evo HE 12
Main PCBUser Interface BoardWiring HarnessCirculation pumpOverheat thermostatMain heat exchanger (flushing needed)
Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.
Call a Gas Safe engineer if…
- You can smell gas or see signs of a leak
- The Evo shows an Emergency or High severity code
- The boiler keeps locking out after repeated resets
- You've tried the DIY checks and the fault hasn't cleared
- There's visible water leaking from the boiler
- The flame is yellow or orange instead of blue